BAW resonator having lateral energy confinement and methods of fabrication thereof

ABSTRACT

Embodiments of a Bulk Acoustic Wave (BAW) resonator in which an outer region of the BAW resonator is engineered in such a manner that lateral leakage of mechanical energy from an active region of the BAW resonator is reduced, and methods of fabrication thereof, are disclosed. In some embodiments, a BAW resonator includes a piezoelectric layer, a first electrode on a first surface of the piezoelectric layer, a second electrode on a second surface of the piezoelectric layer opposite the first electrode, and a passivation layer on a surface of the second electrode opposite the piezoelectric layer, the passivation layer having a thickness (T PA ). The BAW resonator also includes a material on the second surface of the piezoelectric layer adjacent to the second electrode in an outer region of the BAW resonator. The additional material has a thickness that is n times the thickness (T PA ) of the passivation layer.

RELATED APPLICATIONS

This application claims the benefit of provisional patent applicationSer. No. 62/207,702, filed Aug. 20, 2015, the disclosure of which ishereby incorporated herein by reference in its entirety.

FIELD OF THE DISCLOSURE

The present disclosure relates to Bulk Acoustic Wave (BAW) resonatorsand, in particular, to improvement of confinement of mechanical energywithin a BAW resonator.

BACKGROUND

Due to, among other things, their small size, high Q values, and verylow insertion losses at microwave frequencies, particularly those above1.5 Gigahertz (GHz), Bulk Acoustic Wave (BAW) filters have become thefilter of choice for many modern wireless applications. In particular,BAW filters are the filter of choice for many 3^(rd) Generation (3G) and4^(th) Generation (4G) wireless devices. For instance, virtually allLong Term Evolution (LTE) compatible mobile devices operating in LTEfrequency bands above 1.9 GHz utilize BAW filters. For mobile devices,the low insertion loss of the BAW filter provides many advantages suchas, e.g., improved battery life, compensation for higher lossesassociated with the need to support many frequency bands in a singlemobile device, etc.

One example of a conventional BAW resonator 10 is illustrated in FIG.1A. In this example, the BAW resonator 10 is, in particular, a SolidlyMounted Resonator (SMR) type BAW resonator 10. As illustrated, the BAWresonator 10 includes a piezoelectric layer 12 (which is sometimesreferred to as a piezoelectric plate) between a bottom electrode 14 anda top electrode 16. Since the BAW resonator 10 is a SMR type BAWresonator 10, the BAW resonator 10 also includes a reflector 18 (whichis more specifically referred to as a Bragg reflector) that includesmultiple layers 20-28 of alternating materials with varying refractiveindex. In this example, the BAW resonator 10 also includes a Border (BO)ring 30 on the top surface of the top electrode 16 around the peripheryof the top electrode 16. Finally, the BAW resonator 10 includes apassivation layer 32.

In operation, acoustic waves in the piezoelectric layer 12 within anactive region 34 of the BAW resonator 10 are excited by an electricalsignal applied to the bottom and top electrodes 14 and 16. The activeregion 34 is the region of the BAW resonator 10 that is electricallydriven. In other words, the active region 34 is the region of the BAWresonator 10 consisting of, in this example, the bottom electrode 14,the top electrode 16, the portion of the piezoelectric layer 12 betweenthe bottom and top electrodes 14 and 16, and the portion of thereflector 18 below the bottom electrode 14. Conversely, an outer region36 of the BAW resonator 10 is a region of the BAW resonator 10 that isnot electrically driven (i.e., the area outside of the active region34). The frequency at which resonance of the acoustic waves occurs is afunction of the thickness of the piezoelectric layer 12 and the mass ofthe bottom and top electrodes 14 and 16. At high frequencies (e.g.,greater than 1.5 GHz), the thickness of the piezoelectric layer 12 isonly micrometers thick and, as such, the BAW resonator 10 is fabricatedusing thin-film techniques.

Ideally, in order to achieve a high Q value, the mechanical energyshould be contained, or trapped, within the active region 34 of the BAWresonator 10. The reflector 18 operates to prevent acoustic waves fromleaking longitudinally, or vertically, from the BAW resonator 10 intothe substrate (not shown, but below the reflector 18). Notably, in aFilm Bulk Acoustic Resonator (FBAR) type BAW resonator, an air cavity isused instead of the reflector 18, where the air cavity likewise preventsacoustic waves from escaping into the substrate.

While the reflector 18 (or air cavity for a FBAR type BAW resonator)confines mechanical energy within the active region 34 of the BAWresonator 10 in the longitudinal, or vertical, direction, a substantialamount of mechanical energy still leaks laterally from the active region34 of the BAW resonator 10 into the outer region 36 of the BAW resonator10 and then down into the substrate, as illustrated FIG. 1B. Thislateral leakage of mechanical energy at the boundaries of the BAWresonator 10 degrades the Q of the BAW resonator 10. As such, there is aneed for systems and methods for mitigating the loss of mechanicalenergy through lateral dispersion into the outer region 36 of the BAWresonator 10.

SUMMARY

Embodiments of a Bulk Acoustic Wave (BAW) resonator in which an outerregion of the BAW resonator is engineered in such a manner that lateralleakage of mechanical energy from an active region of the BAW resonatoris reduced, and methods of fabrication thereof, are disclosed. In someembodiments, a BAW resonator includes a piezoelectric layer, a firstelectrode on a first surface of the piezoelectric layer, a secondelectrode on a second surface of the piezoelectric layer opposite thefirst electrode on the first surface of the piezoelectric layer, and apassivation layer on a surface of the second electrode opposite thepiezoelectric layer, the passivation layer having a thickness (T_(PA)).The BAW resonator also includes a material on the second surface of thepiezoelectric layer adjacent to the second electrode in an outer regionof the BAW resonator. The outer region of the BAW resonator is a regionoutside of an active region of the BAW resonator. The additionalmaterial has a thickness that is n times the thickness (T_(PA)) of thepassivation layer, wherein n is a value other than 1. In this manner,lateral leakage of the mechanical energy from the active region of theBAW resonator into the outer region of the BAW resonator can be reduced.

In some embodiments, n is within a range of values for which a densityof mechanical energy in the outer region of the BAW resonator is reducedas compared to a density of mechanical energy in the outer region of theBAW resonator when n is equal to 1.

In some embodiments, n is such that the outer region of the BAWresonator and the active region of the BAW resonator are acousticallymatched in such a manner that one or more wavelengths that cause energyleakage into the outer region are not excited in the active region.

In some embodiments, the BAW resonator further includes a Border (BO)ring around a periphery of the active region of the BAW resonator withinor on the second electrode, the BO ring providing a mass loading.

In some embodiments, the BAW resonator further includes a BO ring arounda periphery of the active region of the BAW resonator, and n is suchthat a thickness of the one or more material layers in the outer regionof the BAW resonator is less than or equal to a combined thickness ofthe second electrode, the BO ring, and the passivation layer within theactive region.

In some embodiments, the passivation layer is also on the surface of thepiezoelectric layer adjacent to the second electrode in the outer regionof the BAW resonator, and the one or more material layers in the outerregion consist of the portion of the passivation layer in the outerregion of the BAW resonator such that the thickness of the passivationlayer in the outer region is n times the thickness (T_(PA)) of thepassivation layer in the active region. Further, in some embodiments,the BAW resonator further includes a BO ring around a periphery of theactive region of the BAW resonator, and n is such that a thickness ofthe passivation layer in the outer region of the BAW resonator is lessthan or equal to a combined thickness of the second electrode, the BOring, and the passivation layer within the active region. Further, insome embodiments, the piezoelectric layer is Aluminum Nitride (AlN), thefirst and second electrodes each comprise a Tungsten layer and anAluminum Copper layer, and the passivation layer is Silicon Nitride(SiN).

In some embodiments, the one or more material layers comprise one ormore layers of a material other than a passivation material comprised inthe passivation layer.

Embodiments of a method of fabricating a BAW resonator are alsodisclosed. In some embodiments, the method of fabrication of a BAWresonator includes providing an initial structure comprising apiezoelectric layer and a first electrode on a first surface of thepiezoelectric layer, providing a second electrode on a second surface ofthe piezoelectric layer opposite the first electrode on the firstsurface of the piezoelectric layer, providing a passivation layer on asurface of the second electrode opposite the piezoelectric layer withinan active region of the BAW resonator, the passivation layer having athickness (T_(PA)) within the active region of the BAW resonator, andproviding one or more material layers on the second surface of thepiezoelectric layer adjacent to the second electrode in an outer regionof the BAW resonator, the outer region of the BAW resonator being aregion outside of the active region of the BAW resonator and the one ormore material layers having a thickness that is n times the thickness(T_(PA)) of the passivation layer, wherein n is a value other than 1.

Those skilled in the art will appreciate the scope of the presentdisclosure and realize additional aspects thereof after reading thefollowing detailed description of the preferred embodiments inassociation with the accompanying drawing figures.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

The accompanying drawing figures incorporated in and forming a part ofthis specification illustrate several aspects of the disclosure, andtogether with the description serve to explain the principles of thedisclosure.

FIGS. 1A and 1B illustrate one example of a conventional Bulk AcousticWave (BAW) resonator and lateral leakage of mechanical energy from anactive region of the BAW resonator into an outer region of the BAWresonator;

FIGS. 2A and 2B illustrate a BAW resonator having reduced lateralleakage, as compared to a reference BAW resonator, according to someembodiments of the present disclosure;

FIGS. 3A and 3B illustrate the reduced lateral leakage of the BAWresonator of FIG. 2B as compared to the reference BAW resonator of FIG.2A, for one example implementation;

FIG. 4 illustrates a BAW resonator having reduced lateral leakageaccording to some other embodiments of the present disclosure; and

FIGS. 5A through 5E graphically illustrate a process for fabricating theBAW resonator of either FIG. 2B or FIG. 4 according to some embodimentsof the present disclosure.

DETAILED DESCRIPTION

The embodiments set forth below represent the necessary information toenable those skilled in the art to practice the embodiments andillustrate the best mode of practicing the embodiments. Upon reading thefollowing description in light of the accompanying drawing figures,those skilled in the art will understand the concepts of the disclosureand will recognize applications of these concepts not particularlyaddressed herein. It should be understood that these concepts andapplications fall within the scope of the disclosure and theaccompanying claims.

It should be understood that, although the terms first, second, etc. maybe used herein to describe various elements, these elements should notbe limited by these terms. These terms are only used to distinguish oneelement from another. For example, a first element could be termed asecond element, and, similarly, a second element could be termed a firstelement, without departing from the scope of the present disclosure. Asused herein, the term “and/or” includes any and all combinations of oneor more of the associated listed items.

It should also be understood that when an element is referred to asbeing “connected” or “coupled” to another element, it can be directlyconnected or coupled to the other element or intervening elements may bepresent. In contrast, when an element is referred to as being “directlyconnected” or “directly coupled” to another element, there are nointervening elements present.

It should be understood that, although the terms “upper,” “lower,”“bottom,” “intermediate,” “middle,” “top,” and the like may be usedherein to describe various elements, these elements should not belimited by these terms.

These terms are only used to distinguish one element from another. Forexample, a first element could be termed an “upper” element and,similarly, a second element could be termed an “upper” element dependingon the relative orientations of these elements, without departing fromthe scope of the present disclosure.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the disclosure.As used herein, the singular forms “a,” “an,” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises,”“comprising,” “includes,” and/or “including” when used herein specifythe presence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which this disclosure belongs. It willbe further understood that terms used herein should be interpreted ashaving meanings that are consistent with their meanings in the contextof this specification and the relevant art and will not be interpretedin an idealized or overly formal sense unless expressly so definedherein.

Embodiments of a Bulk Acoustic Wave (BAW) resonator in which an outerregion of the BAW resonator is engineered in such a manner that lateralleakage of mechanical energy from an active region of the BAW resonatoris reduced, and methods of fabrication thereof, are disclosed. Ingeneral, in some embodiments, a thickness of a material in the outerregion of the BAW resonator is such that the outer region of the BAWresonator is acoustically matched to the active region of the BAWresonator in such a manner that wavelengths that cause the lateralleakage of mechanical energy are not excited in the active region. As aresult, there is no leakage of wavelengths excited in the active regioninto oscillation modes in the outer region. In other words, thethickness of the material in the outer region of the BAW resonator isselected such that the extinction coefficient (i.e., the rate ofexponential decay for evanescent waves) associated with the exponentialdecay in the outer region and the imaginary part of the lateraldispersion in the outer region are changed in such a manner that lateralleakage is reduced.

In this regard, FIGS. 2A and 2B illustrate a BAW resonator havingreduced lateral leakage, as compared to a reference BAW resonator,according to some embodiments of the present disclosure. Morespecifically, FIG. 2A illustrates a reference BAW resonator 38. In thisexample, the reference BAW resonator 38 includes a piezoelectric layer40 (which is sometimes referred to as a piezoelectric plate). Thepiezoelectric layer 40 may be any suitable type of piezoelectricmaterial such as, for example, Aluminum Nitride (AlN) or Zinc Oxide(ZnO). Further, the piezoelectric layer 40 may be a single layer ofpiezoelectric material or may include multiple sublayers of the same ordifferent piezoelectric materials.

The reference BAW resonator 38 further includes a bottom electrode 42 ona bottom surface of the piezoelectric layer 40 and a top electrode 44 ona top surface of the piezoelectric layer 40 opposite the bottomelectrode 42. Each of the bottom and top electrodes 42 and 44 may be asingle layer of one material or may include two or more layers of thesame or different materials. For example, in some embodiments, each ofthe bottom and top electrodes 42 and 44 includes a layer of Tungstenimmediately adjacent to the piezoelectric layer 40 and a layer ofAluminum Copper on the Tungsten layer opposite the piezoelectric layer40.

In this example, the reference BAW resonator 38 is a Solidly MountedResonator (SMR) type BAW resonator and, as such, the reference BAWresonator 38 also includes a reflector 46 (which is more specificallyreferred to as a Bragg reflector) that includes multiple alternatinglayers 48, 50, 52, 54, and 56 of alternating materials with varyingrefractive index. In this example, the layers 48, 50, 52, 54, and 56 arealternating layers of Silicon Dioxide (SiO₂) and Tungsten.

In this example, the reference BAW resonator 38 also includes a Border(BO) ring 58. In this example, the BO ring 58 is a “ring” or “frame” ofmaterial that is on the top surface of the top electrode 44 around theperiphery of the top electrode 44. However, the BO ring 58 mayalternative be within the top electrode 44 (i.e., beneath a first metallayer of multiple metal layers forming the top electrode 44 or betweentwo adjacent metal layers in a stack of metal layers forming the topelectrode). As will be appreciated by one of ordinary skill in the art,the BO ring 58 provides mass loading or thickened edge loading, wherethis mass loading avoids acoustic mismatch between an active region andan outer region, providing a smooth transition of propagating waves inthe active region to evanescent waves in the outer region.

Lastly, the reference BAW resonator 38 includes a passivation layer 60on the surface of the reference BAW resonator 38 over both an activeregion 62 and an outer region 64 of the reference BAW resonator 38.While the passivation layer 60 can be of any suitable material, in oneexample, the passivation layer 60 is Silicon Nitride (SiN). For thereference BAW resonator 38, a thickness (T_(PA)) of the passivationlayer 60 is the same both in the active region 62 and in the outerregion 64. Notably, as used herein, the active region 62 is the regionof the reference BAW resonator 38 that is electrically driven which, inthe example of FIG. 2A, is the region consisting of the bottom electrode42, the top electrode 44, the portion of the piezoelectric layer 40between the bottom and top electrodes 42 and 44, and the portion of thereflector 46 beneath the bottom electrode 42. The outer region 64 is theregion of the reference BAW resonator 38 that is not electrically drivenor, in other words, the region of the reference BAW resonator 38 that isoutside of the active region 62.

As illustrated in FIG. 3A, during operation, the reference BAW resonator38 of FIG. 2A exhibits a significant amount of lateral leakage ofmechanical energy from the active region 62 into the outer region 64.This lateral leakage, or lack of lateral confinement, degrades thequality factor (Q) of the reference BAW resonator 38.

FIG. 2B illustrates a BAW resonator 66 with improved lateral confinement(i.e., reduced lateral leakage) of mechanical energy according to someembodiments of the present disclosure. In this example, the BAWresonator 66 includes a piezoelectric layer 68 (which is sometimesreferred to as a piezoelectric plate), bottom and top electrodes 70 and72, a reflector 74 including layers 76, 78, 80, 82, and 84, and a BOring 86, which are exactly the same as the corresponding components ofthe reference BAW resonator 38 and, as such, their details are notrepeated.

Lastly, the BAW resonator 66 includes a passivation layer 88 on thesurface of the BAW resonator 66 over both an active region 90 and anouter region 92 of the BAW resonator 66. Within the active region 90,the passivation layer 88 is exactly the same as the passivation layer 60of the reference BAW resonator 38. Within the active region 90, thepassivation layer 88 has a thickness (T_(PA)), which is equal to that ofthe passivation layer 60 of the reference BAW resonator 38. However, inthe outer region 92, the passivation layer 60 has a thickness ofn×T_(PA), where n≠1 (i.e., the thicknesses of the passivation layers 60and 88 in the outer regions 64 and 92 of the reference BAW resonator 38and the BAW resonator 66, respectively, are not the same). The value ofn is in a range that reduces the lateral leakage of mechanical energyfrom the active region 90 of the BAW resonator 66 into the outer region92 as compared to that of the reference BAW resonator 38 of FIG. 2A.This reduction of lateral leakage is illustrated in FIG. 3B, where FIGS.3A and 3B are graphical illustrations of the results of a simulation ofthe density of mechanical energy throughout the structures of thereference BAW resonator 38 and the BAW resonator 66, respectively.

Suitable values for n may be determined, e.g., by simulation or, in somecases, empirically. However, for most practical implementations,empirical calculations are complex and, as such, simulation will providebetter results.

In general, the value of n is such that the outer region 92 and theactive region 90 of the BAW resonator 66 are acoustically matched suchthat one or more acoustic wavelengths that cause lateral leakage ofmechanical energy from the active region 90 into the outer region 92 arenot excited in the active region 90. As a result, the acoustic couplingbetween the active and outer regions 90 and 92 is mitigated and, assuch, lateral leakage is reduced. In other words, n is selected suchthat the total thickness of the material on the surface of thepiezoelectric layer 68 in the outer region 92 of the BAW resonator 66changes the extinction coefficient associated with the exponential decayin the outer region 92 (as compared to that in the reference BAWresonator 38) and modifies the imaginary part of the lateral dispersionin the outer region 92 in such a manner that lateral leakage is reduced.In some embodiments, the value of n is selected such that the totalthickness of the layers on the surface of the piezoelectric layer 68 inthe outer region 92 is less than or equal to the total thickness of thetop electrode 72, the BO ring 86, and the passivation layer 88 withinthe active region 90. This can be expressed as:T _(PA) _(_) _(OUT) =n*T _(PA) ≦T _(ELEC) +T _(BO) +T _(PA),where T_(PA) _(_) _(OUT) is the thickness of the passivation layer 88 inthe outer region 92, T_(ELEC) is the thickness of the top electrode 72,T_(BO) is the thickness of the BO ring 86, and T_(PA) is the thicknessof the passivation layer 88 in the active region 90. In someembodiments, within the range of 0<n*T_(PA)≦T_(ELEC)+T_(BO)+T_(PA), thevalue of n that provides the best performance (e.g., highest Q) can bedetermined, e.g., via simulation. Thus, the value of n can be said to bea function of the thicknesses of the material stack in the active region90.

In the embodiment of FIG. 2B, the material layers (i.e., the stack ofmaterial layers) in the outer region 92 is modified via the thickness ofthe passivation layer 88 in the outer region 92. However, the presentdisclosure is not limited thereto. Additional or alternative materialsmay be used in the outer region 92 to provide the desired reduction inlateral leakage. In this regard, FIG. 4 illustrates the BAW resonator 66according to another embodiment of the present disclosure. In thisexample, the material layer(s) on the surface of the piezoelectric layer68 in the outer region 92 are generalized as material layer(s) 94. Thematerial layer(s) 94 may be the same material as the passivation layer88 (i.e., the embodiment of FIG. 2B), some other material(s), or anycombination thereof. The material layer(s) 94 has a thickness that isn×T_(PA), as described above. As also described above, the value of n isselected such that lateral leakage is reduced as compared to acorresponding reference BAW resonator (i.e., a BAW resonator that, otherthan the material layer(s) 94, is otherwise exactly the same as the BAWresonator 66).

In the examples of FIGS. 2B and 4, the BAW resonator 66 is a SMR typeBAW resonator. However, the concepts disclosed herein are equallyapplicable to Film Bulk Acoustic Resonator (FBAR) type BAW resonators.

Notably, the use of the material (i.e., the passivation layer 88 havingthickness n×T_(PA) in the embodiment of FIG. 2B or the material layer(s)94 in the embodiment of FIG. 4) in the outer region 92 of the BAWresonator 66 as described herein is to be distinguished fromconventional treatment of type II dispersion stacks. With respect toconventional treatment of type II resonators, mass loading of a BAWresonator is used to move the cutoff frequency of the outer region ofthe BAW resonator to be in the correct position, relative to the cutofffrequency of the active region, in order to trap the waves associatedwith the main thickness extensional wave mode. More specifically, thereare two types of lateral dispersion that a thickness extensional BAWresonator can exhibit, namely, Type I and Type II. Type I, which isnormally exhibited by ZnO based BAW resonators, is where the dispersioncurve of the main thickness extensional wave mode is monotonicallyincreasing from the cut-off frequency, which is defined by the frequencyat which the dispersion curve crosses from real to imaginary. For Type Idispersion, mass loading of the BAW resonator in the outer region is notneeded. More specifically, the top electrode not being present in theouter region is sufficient to have the outside cutoff frequency abovethe active cutoff frequency. This leads to an exponentially decayingmain thickness wave mode in the outer region and, thus, trapping of themain thickness wave mode in the active region.

Type II lateral dispersion, which is normally exhibited by AlN based BAWresonators, is where the dispersion curve of the main thicknessextensional wave mode has a negative slope. For conventional massloading with respect to BAW resonators exhibiting Type II dispersion,the idea is to trap the small kx wave modes on the negative part of themain thickness branch. For that, the cutoff frequency of the outerregion of the BAW resonator must be below the cutoff frequency of theactive region to ensure that there is no propagating wave modeassociated with the main thickness wave mode in the outer region.Therefore, conventional mass loading is used to move the cutofffrequency of the outer region of the BAW resonator below the cutofffrequency of the active region.

The problem with such conventional treatment of type II resonator isthat it does not take into account the presence of any other higher kxbranches. In practice, other higher kx branches must be taken intoaccount to minimize lateral leakage.

In the present disclosure, depending on the particular implementation,the BAW resonator 66 may exhibit either Type I or Type II dispersion.Regardless of the dispersion type exhibited by the BAW resonator 66,unlike conventional treatment of type II resonators which moves thecutoff frequency of the outer region to a desired point relative to thecutoff frequency of the active region (i.e., higher than the cutofffrequency of the active region for Type I dispersion or lower than thecutoff frequency of the active region for Type II dispersion), thematerial stack in the outer region 92 of the BAW resonator 66 isengineered, as described above, in such a manner that the extinctioncoefficient associated with the exponential decay in the outer region 92is changed and the imaginary part of the lateral dispersion in the outerregion 92 is modified. By doing so, the way in which waves decay in theouter region 92 can be controlled to reduce lateral leakage. Forexample, for an embodiment of the BAW resonator 66 exhibiting Type IIdispersion, the material stack in the outer region 92 is modified toimprove energy trapping by modifying the imaginary part of thedispersion curve in the outer region 92. While engineering the materialstack in the outer region 92 in this manner will alter the cutofffrequency of the outer region 92, the altered cutoff frequency of theouter region 92 will not necessarily be lower than the cutoff frequencyof the active region 90, which is contrary to the conventional massloading for Type II dispersion.

FIGS. 5A through 5E graphically illustrate a process for fabricating theBAW resonator of either FIG. 2B or FIG. 4 according to some embodimentsof the present disclosure. As illustrated, the process begins with aninitial structure that includes the piezoelectric layer 68, the bottomelectrode 70, and, in this example, the reflector 74. Note, however,that the initial structure may vary depending on the particularimplementation. The initial structure may be fabricated using anyappropriate process.

Next, as illustrated in FIG. 5B, the top electrode 72 is provided on(e.g., formed or deposited on) the surface of the piezoelectric layer 68opposite the bottom electrode 70. Then, in this example, the BO ring 86is provided on the surface of the top electrode 72 opposite thepiezoelectric layer 68 around the periphery of the active region 90, asillustrated in FIG. 5C. Lastly, in the embodiment of FIG. 2B, thepassivation layer 88 is provided (e.g., formed or deposited) on thesurface of the BAW resonator 66 in both the active region 90 and theouter region 92 such that the thickness of the passivation layer 88 inthe outer region 92 is n times the thickness (T_(PA)) of the passivationlayer 88 within the active region 90, as illustrated in FIG. 5D. Moregenerally, the passivation layer 88 is provided in the active region 90and the one or more material layers 94 are provided in the outer region92 according to the embodiment of FIG. 4, as illustrated in FIG. 5E.

Those skilled in the art will recognize improvements and modificationsto the preferred embodiments of the present disclosure. All suchimprovements and modifications are considered within the scope of theconcepts disclosed herein and the claims that follow.

What is claimed is:
 1. A Bulk Acoustic Wave (BAW) resonator, comprising:a piezoelectric layer; a first electrode on a first surface of thepiezoelectric layer; a second electrode on a second surface of thepiezoelectric layer opposite the first electrode on the first surface ofthe piezoelectric layer; a passivation layer on a surface of the secondelectrode opposite the piezoelectric layer within an active region ofthe BAW resonator, the passivation layer having a thickness (T_(PA))within the active region of the BAW resonator; and one or more materiallayers on the second surface of the piezoelectric layer adjacent to thesecond electrode in an outer region of the BAW resonator, the outerregion of the BAW resonator being a region outside of the active regionof the BAW resonator and the one or more material layers having athickness that is n times the thickness (T_(PA)) of the passivationlayer within the active region, wherein: n is a value other than 1; andn is within a range of values for which a density of mechanical energyin the outer region of the BAW resonator is reduced as compared to adensity of mechanical energy in the outer region of the BAW resonatorwhen n is equal to
 1. 2. The BAW resonator of claim 1 wherein the one ormore material layers comprise one or more layers of a material otherthan a passivation material comprised in the passivation layer.
 3. TheBAW resonator of claim 1 wherein n is such that the outer region of theBAW resonator and the active region of the BAW resonator areacoustically matched in such a manner that one or more wavelengths thatcause energy leakage into the outer region are not excited in the activeregion.
 4. The BAW resonator of claim 1 further comprising a border ringaround a periphery of the active region of the BAW resonator within oron the second electrode, the border ring providing a mass loading. 5.The BAW resonator of claim 1 further comprising a border ring around aperiphery of the active region of the BAW resonator, and n is such thata thickness of the one or more material layers in the outer region ofthe BAW resonator is less than or equal to a combined thickness of thesecond electrode, the border ring, and the passivation layer within theactive region.
 6. The BAW resonator of claim 1 wherein the passivationlayer is also on the second surface of the piezoelectric layer adjacentto the second electrode in the outer region of the BAW resonator, andthe one or more material layers in the outer region is formed by aportion of the passivation layer in the outer region of the BAWresonator such that a thickness of the passivation layer in the outerregion is the n times the thickness (T_(PA)) of the passivation layer inthe active region.
 7. The BAW resonator of claim 6 further comprising aborder ring around a periphery of the active region of the BAWresonator, and n is such that a thickness of the passivation layer inthe outer region of the BAW resonator is less than or equal to acombined thickness of the second electrode, the border ring, and thepassivation layer within the active region.
 8. The BAW resonator ofclaim 7 wherein the piezoelectric layer is Aluminum Nitride (AlN), thefirst and second electrodes each comprise a Tungsten layer and anAluminum Copper layer, and the passivation layer is Silicon Nitride(SiN).
 9. A BAW resonator comprising: a piezoelectric layer; a firstelectrode on a first surface of the piezoelectric layer; a secondelectrode on a second surface of the piezoelectric layer opposite thefirst electrode on the first surface of the piezoelectric layer; apassivation layer on a surface of the second electrode opposite thepiezoelectric layer within an active region of the BAW resonator, thepassivation layer having a thickness (T_(PA)) within the active regionof the BAW resonator; and one or more material layers on the secondsurface of the piezoelectric layer adjacent to the second electrode inan outer region of the BAW resonator, the outer region of the BAWresonator being a region outside of the active region of the BAWresonator and the one or more material layers having a thickness that isn times the thickness (T_(PA)) of the passivation layer within theactive region, wherein: n is a value other than 1; and n is such thatthe outer region of the BAW resonator and the active region of the BAWresonator are acoustically matched in such a manner that one or morewavelengths that cause energy leakage into the outer region are notexcited in the active region.
 10. A method of fabricating a BAWresonator, comprising: providing an initial structure comprising apiezoelectric layer and a first electrode on a first surface of thepiezoelectric layer; providing a second electrode on a second surface ofthe piezoelectric layer opposite the first electrode on the firstsurface of the piezoelectric layer; providing a passivation layer on asurface of the second electrode opposite the piezoelectric layer withinan active region of the BAW resonator, the passivation layer having athickness (T_(PA)) within the active region of the BAW resonator; andproviding one or more material layers on the second surface of thepiezoelectric layer adjacent to the second electrode in an outer regionof the BAW resonator, the outer region of the BAW resonator being aregion outside of the active region of the BAW resonator and the one ormore material layers having a thickness that is n times the thickness(T_(PA)) of the passivation layer within the active region, wherein: nis a value other than 1; and n is such that the outer region of the BAWresonator and the active region of the BAW resonator are acousticallymatched in such a manner that one or more wavelengths that cause energyleakage into the outer region are not excited in the active region. 11.A method of fabricating a Bulk Acoustic Wave (BAW) resonator,comprising: providing an initial structure comprising a piezoelectriclayer and a first electrode on a first surface of the piezoelectriclayer; providing a second electrode on a second surface of thepiezoelectric layer opposite the first electrode on the first surface ofthe piezoelectric layer; providing a passivation layer on a surface ofthe second electrode opposite the piezoelectric layer within an activeregion of the BAW resonator, the passivation layer having a thickness(T_(PA)) within the active region of the BAW resonator; and providingone or more material layers on the second surface of the piezoelectriclayer adjacent to the second electrode in an outer region of the BAWresonator, the outer region of the BAW resonator being a region outsideof the active region of the BAW resonator and the one or more materiallayers having a thickness that is n times the thickness (T_(PA)) of thepassivation layer within the active region, wherein: n is a value otherthan 1; and n is within a range of values for which a density ofmechanical energy in the outer region of the BAW resonator is reduced ascompared to a density of mechanical energy in the outer region of theBAW resonator when n is equal to
 1. 12. The method of claim 11 wherein nis such that the outer region of the BAW resonator and the active regionof the BAW resonator are acoustically matched in such a manner that oneor more wavelengths that cause energy leakage into the outer region arenot excited in the active region.
 13. The method of claim 11 furthercomprising providing a border ring around a periphery of the activeregion of the BAW resonator within or on the second electrode, theborder ring providing a mass loading.
 14. The method of claim 11 furthercomprising providing a border ring around a periphery of the activeregion of the BAW resonator, and n is such that a thickness of the oneor more material layers in the outer region of the BAW resonator is lessthan or equal to a combined thickness of the second electrode, theborder ring, and the passivation layer within the active region.
 15. Themethod of claim 11 wherein providing the passivation layer comprisesproviding the passivation layer such that the passivation layer is alsoon the second surface of the piezoelectric layer adjacent to the secondelectrode in the outer region of the BAW resonator, and the one or morematerial layers in the outer region is formed by a portion of thepassivation layer in the outer region of the BAW resonator such that thethickness of the passivation layer in the outer region is the n timesthe thickness (T_(PA)) of the passivation layer in the active region.16. The method of claim 15 further comprising providing a border ringaround a periphery of the active region of the BAW resonator, and n issuch that a thickness of the passivation layer in the outer region ofthe BAW resonator is less than or equal to a combined thickness of thesecond electrode, the border ring, and the passivation layer within theactive region.
 17. The method of claim 16 wherein the piezoelectriclayer is Aluminum Nitride (AlN), the first and second electrodes eachcomprise a Tungsten layer and an Aluminum Copper layer, and thepassivation layer is Silicon Nitride (SiN).
 18. The method of claim 11wherein the one or more material layers comprise one or more layers of amaterial other than a passivation material comprised in the passivationlayer.